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I've just had to do something simliar and setting both size to Rect.W1dth()/2 during creation worked? ie both my tree and my list view were initially the same size! heres the code I used (which does look much like you were doing anyway!
m_HorizontalSplitter.CreateStatic( m_pFrameWnd,1,2);
m_HorizontalSplitter.CreateView( 0,0,
RUNTIME_CLASS( ExclTreeView ),
CSize(FrameRect.Width()/2,10),
NULL);
m_HorizontalSplitter.CreateView( 0,1,
RUNTIME_CLASS( ExclListView ),
CSize(FrameRect.Width()/2,10),
NULL);

NOTE:
1. You can adjust the sizes of views by the same way in OnSize()
2. In this way, you can't see the splitter bar.But I think you needn't to change views' size
by hand, right? If you really need to adjust size by hand, you can follow the following code.

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